I'll typically take as long as I need to find a car I'm happy with. I usually try to scope out the aisle from the bus, if there is one, otherwise I'll go up and down each aisle. If there's a long wait at the exit (usually happens in Seattle), I'll watch for what's being pulled around and check out the options in the remaining cars. I'm rarely rushed on my itinerary and usually have to pound some miles, so it's worth it to find a car that has the space I need, and is a reasonably nice or "cool" car.

Having said that, at LAX I haven't lucked out on selection, so at that location I do hurry off the bus and usually grab the first decent option.